Magnetic recording medium having a magnetic layer and a partially-filled liquid lubricant-containing coating thereunder

ABSTRACT

A magnetic recording medium comprising a nonmagnetic substrate, a liquid lubricant-containing coating layer having a porosity (U) formed on the substrate, and a magnetic layer having a porosity (M) formed directly on the coating layer. The ratio of porosity (U/M) between the coating and magnetic layers is between 0.75 and 1.25. The volume percentage of the lubricant contained in the pores of the coating layer based on the whole pore volume in the coating layer is from 35 to 85%. This improves the durability of the magnetic layer.

FIELD OF THE INVENTION

The present invention relates generally to a magnetic recording medium,and more specifically, to a magnetic recording medium comprising anonmagnetic substrate, a liquid lubricant-containing coating layerformed on the substrate, and a magnetic layer formed directly on thecoating layer.

DESCRIPTION OF THE RELATED ART

There is a seemingly neverending demand for increasing the storagecapacity, and thus usually, the storage density, of removable storagemedia. One approach for accomplishing this is to reduce the thickness ofthe magnetic recording layer. However, when the thickness of themagnetic layer is reduced to less than 1 μm, some practical problemssuch as decrease of durability of the magnetic layer arise. This isbecause, since the thickness of the magnetic layer which directlycontacts a magnetic head is decreased, the magnetic layer cannot have avolume of pores sufficient to create a lubricant reservoir capable ofimproving the durability of the magnetic layer by the improvement oftribologic properties such as the decrease of a coefficient of frictionin the case of contact of the magnetic layer to the magnetic head andincrease of wear resistance.

One type of magnetic recording media for solving such problems comprisesa nonmagnetic support or substrate, a coating layer formed on thesubstrate, and a magnetic layer formed directly on the coating layer.The coating layer can contain an amount of a lubricant necessary for themagnetic layer. In this medium, the coating layer may be a magneticlayer which comprises a magnetic material and a binder, and which isformed independently from the above magnetic layer, or a nonmagneticlayer comprising a nonmagnetic powder and a binder.

Examples of such magnetic recording media follow.

(1) Japanese Patent Kokai Publication No. 175922/1986 is one of theearliest patent applications which tried to solve the above problems bydifferentiating a porosity between the coating layer and the magneticlayer formed directly on the coating layer.

This Kokai publication discloses a magnetic recording medium comprisinga nonmagnetic support a nonmagnetic undercoat layer as a coating layerformed on the nonmagnetic support, and a magnetic layer formed directlyon the undercoat layer, characterized in that a porosity (M) of themagnetic layer is smaller than a porosity (U) of the undercoat layer.

In the working example of this Kokai publication, there is disclosed amagnetic recording medium in which the porosity of the magnetic layer is20 vol. % and that of the undercoat layer is 45 vol. %.

Magnetic recording media which intend to achieve the same object asJapanese Kokai Publication No. 175922/1986 by making the porosity of thecoating layer larger than that of the magnetic layer on the coatinglayer are disclosed also in Japanese Kokai Publication Nos. 260124/1990and 182183/1993.

A magnetic recording medium disclosed in Japanese Kokai Publication No.260124/1990 has a first magnetic layer as a coating layer and a secondmagnetic layer formed directly on the first magnetic layer and ischaracterized by a porosity of the first magnetic layer of from 0.25 to0.55 and a porosity of the second magnetic layer of from 0.10 to lessthan 0.25.

A magnetic recording medium disclosed in Japanese Patent KokaiPublication No. 182183/1993 is characterized by a porosity of the firstmagnetic layer of from 30 to 50% and a porosity for the second magneticlayer of from 10 to 30% by weight.

(2) In contrast to the above recording media, Japanese Kokai PublicationNo. 293718/1988 discloses a magnetic recording medium which achieves theabove object by making the porosity of the coating layer smaller thanthat of the magnetic layer formed on the coating layer. The magneticrecording medium comprises a first magnetic layer as a coating layer anda second magnetic layer formed directly on the first magnetic layer, andis characterized by a ratio of porosity (M₂) of the second magneticlayer to a porosity (M₁) of the first magnetic layer (M₂ /M₁) of from1.1 to 2.

(3) Japanese Kokai Publication No. 168439/1994 discloses a magneticrecording medium which achieves the same object as in the media of (1)by differentiating contents of a fatty acid and a fatty acid ester as alubricant between the coating layer and the magnetic layer on thecoating layer.

The magnetic recording medium disclosed in this Kokai has a nonmagneticlayer as a coating layer and a magnetic layer formed directly on thenonmagnetic layer and is characterized by a porosity of the nonmagneticlayer of at least 15%, a porosity of the magnetic layer of 15% or less,and a content of the aliphatic acid in the magnetic layer larger thanthat in the nonmagnetic layer, while a content of the fatty acid esterin the non-magnetic layer is larger than that in the magnetic layer.

(4) Japanese Kokai Publication No. 4854/1994 discloses a magneticrecording medium which achieves the same object as those in (1) byseparating a coating layer into two sublayers. That is, the disclosedmagnetic recording medium has a nonmagnetic layer as a coating layer anda magnetic layer formed on the nonmagnetic layer and is characterized inthat the nonmagnetic layer is divided in two sublayers, and the porosityof a lower nonmagnetic sublayer on the nonmagnetic support side islarger than that of the upper nonmagnetic sublayer.

SUMMARY OF THE INVENTION

However, the durability of the magnetic layer which directly contactsthe magnetic head may not be sufficiently improved only by controllingthe porosities of the magnetic layer and the coating layer in thespecific ranges as disclosed in the prior art. In particular, with ahigh density magnetic recording medium, a magnetic layer of which shouldhave a decreased thickness, for example, 0.5 μm or less, it is verydifficult to improve the durability of the magnetic layer even by theabove prior art. An object of the present invention is to provide amagnetic recording medium which improves both the durability andcarrier-to-noise ratio characteristics of the magnetic layer.

To solve the above problem, the present invention provides a magneticrecording medium comprising a nonmagnetic support, a liquidlubricant-containing coating layer formed on the nonmagnetic support,and a magnetic layer formed directly on the coating layer. The magneticlayer has a porosity (M) and the coating layer has a porosity (U). Theratio of the porosity (M) of the magnetic layer to the porosity (U) ofsaid coating layer is from 0.75 to 1.25, i.e., 0.75≦M/U≦1.25. The volumepercentage of the liquid lubricant contained in the pores of the coatinglayer based on the whole pore volume of the coating layer is from 35 to85%. This improves the durability of the magnetic layer.

DETAILED DESCRIPTION

When the ratio of the porosities (M/U) is less than 0.75, since asufficient amount of the liquid lubricant is not supplied from thecoating layer to the magnetic layer, the durability of the magneticlayer is deteriorated. When the ratio of the porosities (M/U) exceeds1.25, since an excessive amount of the liquid lubricant is present onthe surface of the magnetic layer, a coefficient of friction between themagnetic head and the magnetic layer (namely, a torque when the magnetichead runs over the magnetic layer) is increased, so that the durabilityof the magnetic layer is deteriorated. In addition, the increase of thecoefficient of friction leads to failure of the magnetic head, namelyunstable recording and reproducing. From this view point, the ratio ofporosities (M/U) is preferably from 0.9 to 1.2.

When the percentage of the volume of the liquid lubricant contained inthe pores of the coating layer based on the whole pore volume in thecoating layer is less than 35%, since a sufficient amount of the liquidlubricant is not supplied from the coating layer to the magnetic layer,the durability of the magnetic layer is deteriorated. When thispercentage exceeds 85%, since an excessive amount of the liquidlubricant is present on the surface of the magnetic layer, thedurability of the magnetic layer is deteriorated and further runningfailure of the magnetic head is caused as described above. From thisview point, this percentage is preferably from 50 to 80%.

The control of the amount of liquid lubricant present in the magneticlayer and on the magnetic layer surface tends to be particularlydifficult when the magnetic recording medium is in the form of a floppydisc, as will be explained below.

The floppy disc is assembled by loading a magnetic recording mediumwhich has been punched in a disk shape in a container made of a hard orsoft plastic (called a "shell" or "jacket") and used. On inner surfacesof the container, respective cleaning members (called "liners") areprovided to contact the magnetic layer surface and clean it. Thecleaning member is made of nonwoven fabric to wipe out particles or dustwhich may cause errors without damaging the magnetic layer surface.However, the nonwoven fabric wipes out the liquid lubricant togetherwith the particles or dusts, and the liquid lubricant, once absorbed inthe nonwoven fabric, cannot be used to lubricate the magnetic layersurface.

Accordingly, since the excessive amount of liquid lubricant present onthe magnetic layer surface is absorbed by the nonwoven fabric and theamount of liquid lubricant to be maintained by the coating layer and themagnetic layer is decreased, the durability of the magnetic layer isdeteriorated. The present invention makes it possible to suitablycontrol the amount of liquid lubricant present in the magnetic layer andon the magnetic layer surface and, in turn, to improve the durability ofthe floppy disc.

Now, the components of the magnetic recording medium of the presentinvention will be further explained.

Coating layer

The coating layer of the present invention may be (i) a magnetic layercomprising a magnetic material and a binder, or (ii) a nonmagnetic layercomprising a nonmagnetic powder and a binder.

For the magnetic material, any of the conventional magnetic materialssuch as a metal magnetic material comprising pure iron, iron oxidemagnetic materials, for example, cobalt-containing gamma-iron oxide,barium ferrite and iron carbide may be used. As the nonmagnetic powder,any of the conventional nonmagnetic powders such as aluminum oxide,chromium oxide, titanium monoxide, titanium dioxide, tin oxide,alpha-iron oxide, graphite, silicon carbide, calcium carbonate, bariumsulfate, silicon dioxide, or their mixtures may be used. Among them,titanium dioxide, tin oxide, and alpha iron oxide are particularlypreferred.

Since such nonmagnetic powder is easily dispersed in the binder (as willbe explained below), control of the porosity of the coating layer can beeasily achieved. Since such inorganic powder is available in arelatively minute size and in a needle shape, and because the use ofsuch minute needle shape inorganic powder makes it easy to smooth thesurface of the coating layer, the surface of the magnetic layer which isformed on the coating layer is smoothed and the magnetic recordingcharacteristics are improved.

An average particle size of such nonmagnetic powder is preferably from0.01 to 0.5 μm, more preferably from 0.05 to 0.3 μm. When thenonmagnetic powder particle has the needle shape, its acicular ratio ispreferably in the range between 3 and 20, more preferably in the rangebetween 5 and 10.

In a preferred embodiment, a mixture of an inorganic powder and carbonblack is used as the nonmagnetic powder. Since the addition of carbonblack prevents static electrification on the surface of the magneticlayer formed on the coating layer and therefore the adhesion of foreignparticles such as dust caused by the static electricity, the generationof errors and damage of the magnetic layer surface due to the preventionof contact between the magnetic head and the magnetic layer surface areprevented.

A weight ratio of the nonmagnetic powder to the binder is preferablyfrom 85:15 to 65:35. When this weight ratio is outside this range,control of the porosity of the coating layer is difficult and, as aresult, the control of the ratio of porosities (M/U) tends to becomedifficult. From this viewpoint, a preferred range of this weight ratiois from 80:20 to 70:30.

As the binder, any of the binder resins used in the magnetic recordingmedium such as polyurethane, polyvinyl chloride, polyvinylidenechloride, epoxy resin, phenol resin, acrylic resin, cellulose resin andpolyester may be used. The binder may contain a hardener such as anisocyanate to improve the elasticity or solvent resistance of thecoating layer. In this case, the weight ratio of the nonmagnetic powderto the binder is calculated using a total weight of the binder and thehardener as a weight of the "binder."

The coating layer is formed by coating a paint, which is prepared byadding various additives such as a solvent, an abrasive, an antistaticagent, a fungicide, an antioxidant and so on as desired to the abovematerials, on the nonmagnetic support with a conventional coating means.

The paint of the coating layer may be prepared using a known kneadingapparatus or dispersion apparatus, such as a kneader, a planetary mixer,an extruder, a homogenizer, a sand mill, a ball mill, or an attritor.The paint of the coating layer may be coated on the support using aconventional coating means, such as an air doctor coater, a bladecoater, an air knife coater, a squeeze coater, a reverse roll coater, agravure coater, a kiss coater, a spray coater, or a die coater.

The coating layer is preferably formed as a single layer for thesimplification of the production step, while it may be formed as amultilayer comprising the above magnetic layer (i) and/or the abovenonmagnetic layer (ii).

A thickness of the coating layer is usually 5 μm or less. To readilysupply the liquid lubricant (the details of which will be explainedbelow) from the coating layer to the surface of the magnetic layerformed thereon, the thickness of the coating layer is preferably 3 μm orless, more preferably from 1 to 2.5 μm.

To include the liquid lubricant in the coating layer, the liquidlubricant is either directly added to the coating layer mixture and thenthe mixture is coated, or the coating layer is formed and then a liquidcontaining the lubricant is applied thereon.

The porosity of the coating layer can be easily controlled by selectingthe conditions such as (a) calendering conditions (the porosity beinglarger as a pressure or a temperature being lower), (b) the size andshape of the nonmagnetic powder particles (the porosity being larger forlarger particles, or the porosity being made larger by the needle-shapedparticles than the spherical particles), (c) the content of the binderin relation to the nonmagnetic powder (a smaller amount of the bindermaking the porosity larger), (d) kneading conditions in the preparationof the mixture (a larger solid content in the kneading step or a largerratio of the powder to the resin making the porosity larger), and (e) adrying rate (quicker drying making the porosity larger).

Magnetic layer

The magnetic layer in the present invention can be formed by the samemanner as the above-described coating layer comprising the magneticmaterial. It is formed directly on the coating layer. This means that noother layer such as an adhesive layer comprising a resin is presentbetween the coating layer and the magnetic layer, whereby the liquidlubricant is well supplied from the coating layer to the magnetic layerand then the durability of the magnetic layer is improved.

The thickness of the magnetic layer is usually 3 μm or less. To improvethe electromagnetic conversion characteristics as the high densitymagnetic recording medium, the thickness is preferably less than 1 μm,more preferably 0.5 μm or less. The magnetic layer may be formed as amultilayer in accordance with requirements arising from a magneticrecording system in which the magnetic recording medium is used. Theporosity of the magnetic layer can be controlled in the same manner asin the case of the coating layer.

Nonmagnetic support

For the nonmagnetic support, any of conventionally used nonmagneticsupports such as flexible supports, for example, a paper sheet, aplastic film, etc., and nonflexible supports, for example, a nonmagneticmetal, a ceramic, etc., may be used. When the magnetic recording mediumof the present invention is used as a floppy disc, the flexible supportis used. For the plastic film, a single layer film or a multilayer filmof plastics such as polyethylene terephthalate, polyethylenenaphthalate, polybutyrene terephthalate, polyamide, polyimide,polyphenylene sulfide, polyether ketone, polyether ester, polyethersulfone, polyether imide, polysulfone, polyarylate, and the like may beused.

Liquid lubricant

As used herein, the term "liquid lubricant" means a lubricant which isin the liquid state at a normal temperature (about 25° C.). For theliquid lubricant, fatty acid esters, unsaturated fatty acids, fattyacids having a side chain, silicone oils, paraffins, and the like arepreferably used. They may be used independently or as a mixture. Theliquid lubricant may contain a fatty acid which is in the solid state atthe normal temperature (about 25° C.) such as stearic acid, palmiticacid or myristic acid. The fatty acid may be used by dissolving ordispersing it in the liquid lubricant.

When the coating layer is the nonmagnetic layer comprising thenonmagnetic powder and the binder, the liquid lubricant is preferablypresent in an amount from 5 to 15 parts by weight, and more preferablyfrom 8 to 11 parts by weight, per 100 parts by weight of the nonmagneticpowder. When the coating layer is the magnetic layer, the amount of theliquid lubricant is preferably from 3 to 10 parts by weight per 100parts by weight of the magnetic material in the coating layer.

The present invention will be explained further in detail by thefollowing non-limiting Examples and Comparative Examples.

In the following Examples and Comparative Examples, the pore volume andthe porosity were measured as follows:

Before the formation of the magnetic layer or after the removal of themagnetic layer, a maximum volume of the liquid lubricant, which can beabsorbed by the layer which does not contain any lubricant, per unitvolume of the coating layer is measured and used as a "pore volume."With the coating layer or the magnetic layer, the "pore volume" ismeasured in the same manner as above. Then, the pore volume is dividedby an apparent volume of the layer (which is calculated from an apparentthickness and a surface area of the sample) and used as a "porosity."

The details of the materials used in the Examples and ComparativeExamples are as follows:

Alpha iron oxide: DNS-235 (manufactured by Toda Industries Ltd. Anaverage particle size (longer axis)=0.25 μm. An acicular ratio=7.7)

Polyurethane resin: TM-4 (manufactured by Toyobo Co., Ltd. A polyesterpolyurethane resin having a sodium sulfonate salt group in a molecule)

Vinyl chloride resin: MR-113 (manufactured by Nippon Zeon Co., Ltd. Avinyl chloride resin having a sodium sulfonate salt group, an epoxygroup and a hydroxyl group in a molecule)

Carbon black: Ketjen Black EC 600 DJ (supplied by Lion Co., Ltd. Oilabsorption=495 ml/100 g)

Oleic acid: Lunac-OA (manufactured by Kao Co., Ltd.)

Isocetyl stearate: ICS-R (manufactured by Kokyu Alkol Co., Ltd.)

Isocyanate (hardener): SBU-0856 (manufactured by Sumitomo Bayer UrethaneCo., Ltd.)

Metal magnetic material: HM-55 (manufactured by Dowa Mining Co., Ltd. Ametal magnetic material comprising pure iron and having a BET of 45 m²/g)

Alumina (abrasive): HIT-50 (manufactured by Sumitomo Chemical Co., Ltd.BET=8.4 m² /g)

EXAMPLES 1-4

The materials used in the coating layer for Examples 1-4 as shown below:

    ______________________________________                                        Undercoat (coating) layer                                                     composition (wt. parts)                                                                       Ex. 1  Ex. 2    Ex. 3                                                                              Ex. 4                                    ______________________________________                                        Alpha-iron oxide                                                                              100    100      100  100                                      Polyurethane resin                                                                            14     12       14   14                                       Vinyl chloride resin                                                                          14     12       14   14                                       Carbon black    5      5        5    5                                        Oleic acid      1      1        1    1                                        Isocetyl stearate                                                                             8      9        7    11                                       Isocyanate      10     9        10   10                                       Methyl ethyl ketone                                                                           213    205      211  217                                      Cyclohexanone   71     69       70   72                                       Toluene         71     69       70   72                                       ______________________________________                                    

The composition of the magnetic layer for Examples 1-4 was:

    ______________________________________                                        Magnetic layer composition                                                                      Parts by weight                                             ______________________________________                                        Metal magnetic material                                                                         100                                                         Alumina           8                                                           Polyurethane resin                                                                              10                                                          Vinyl chloride resin                                                                            10                                                          Oleic acid        2                                                           Isocetyl stearate 2                                                           Isocyanate        7                                                           Methyl ethyl ketone                                                                             194                                                         Cyclohexanone     65                                                          Toluene           65                                                          ______________________________________                                    

Each of the above compositions was dispersed in a sand mill usingzirconia beads as milling media to prepare an undercoat paint and amagnetic paint. On each of the surfaces of a plastic film having athickness of 62 μm (polyethylene terephthalate, Product No. C-196,manufactured by Teijin Co., Ltd.), the undercoat paint was coated bygravure coating and crosslinked. Then, the magnetic coating mixture wascoated on the respective undercoat layer by the same coating method,calendered, and crosslinked. The thickness of each undercoat layer was2.0 μm, and that of each magnetic layer was 0.5 μm.

The Table shows a ratio of the porosity of the magnetic layer to that ofthe coating layer M/U), the percentage of the volume of liquid lubricant(isocetyl stearate) contained in the pores of the coating layer based ona whole pore volume in the coating layer (X %), and the runningdurability and electromagnetic conversion characteristics of themagnetic recording medium which were measured as follows:

                  TABLE                                                           ______________________________________                                        Example        X     Running durability                                                                        Electromagnetic                              No.    M/U     %     (passes)    conversion (C/N) (dB)                        ______________________________________                                        Ex. 1  1.1     60    30 × 10.sup.6                                                                       +1.5                                         Ex. 2  0.9     60    25 × 10.sup.6                                                                       +1.5                                         Ex. 3  1.1     50    28 × 10.sup.6                                                                       +1.5                                         Ex. 4  1.1     80    30 × 10.sup.6                                                                       +1.5                                         C. Ex. 1                                                                             1.3     60     5 × 10.sup.6                                                                       +0                                           C. Ex. 2                                                                             0.7     60     6 × 10.sup.6                                                                       +1.5                                         C. Ex. 3                                                                             1.1     30    10 × 10.sup.6                                                                       +1.5                                         C. Ex. 4                                                                             1.1     90     3 × 10.sup.6                                                                       Could not be measured                                                         due to running failure                       ______________________________________                                    

The magnetic recording medium which had been produced as above waspunched in the form of a 3.5 inch (89 mm) disc and contained in aconventionally used shell (a hard plastic container for a floppy disc).With this floppy disc, the running durability and electromagneticconversion characteristics were measured.

On inner surfaces of the shell, respective liners made of nonwovenfabric (the above cleaning members) were provided so that the linerscontacted the respective surfaces of the magnetic disc. The nonwovenfabric consisted of a textile blend of 70 parts by weight of rayonfibers and 30 parts by weight of polyester fibers.

The running durability was evaluated by continuously running the floppydisc while contacting the magnetic heads to both magnetic surfaces ofthe floppy disc using a 10 MB FD Drive FD-1331 manufactured by NEC), andcounting the number of passes until scratches visible to the naked eyewere found on either one of the magnetic surfaces.

Regarding the electromagnetic conversion characteristics of the floppydisc, a C/N (a ratio of a carrier signal (C) of 600 kHz to a noise (N)near the carrier signal) was measured. The measured values wereexpressed as relative values with the C/N of the floppy disc produced inComparative Example being 0 dB. The larger plus value of the C/N meansthe better electromagnetic conversion characteristics.

COMPARATIVE EXAMPLES 1-4

Comparative Examples 1-4 were prepared in the same manner as Examples1-4, except that the following undercoat compositions were used:

    ______________________________________                                        Undercoat layer                                                                              Comparative Example No.                                        composition (wt. parts)                                                                      1       2        3     4                                       ______________________________________                                        Alpha-iron oxide                                                                             100     100      100   100                                     Polyurethane resin                                                                           17      9        14    14                                      Vinyl chloride resin                                                                         17      9        14    14                                      Carbon black   5       5        5     5                                       Oleic acid     1       1        1     1                                       Isocetyl stearate                                                                            6       10       4     12                                      Isocyanate     12      7        10    10                                      Methyl ethyl ketone                                                                          221     197      207   218                                     Cyclohexanone  74      66       69    73                                      Toluene        74      66       69    73                                      ______________________________________                                    

What is claimed is:
 1. A magnetic recording medium comprising anonmagnetic substrate, a liquid lubricant-containing coating layerformed on the substrate, and a magnetic layer formed directly on thecoating layer, wherein the maximum amount of liquid lubricant which canbe absorbed by a layer per unit volume of the layer is defined by U forthe lubricant-containing coating layer and M for the magnetic layer,wherein the amount of liquid lubricant actually present within thelubricant-containing coating layer is in the range from 50% to 80% of U.0.9≦M/U≦1.2, and the magnetic layer has a thickness of 0.5 μm or less.2. The medium of claim 1, wherein the lubricant-containing coating layeris a nonmagnetic layer comprising a nonmagnetic powder and a binder, thelubricant-containing coating layer having a weight ratio of thenonmagnetic powder to the binder from 85:15 to 65:35.
 3. The medium ofclaim 2, wherein an amount of said liquid lubricant in thelubricant-containing coating layer is from 5 to 15 parts by weight per100 parts by weight of the non-magnetic powder.
 4. The medium of claim3, wherein the magnetic layer has a thickness of 0.5 μm or less.
 5. Themedium of claim 2, wherein the magnetic layer has a thickness of 0.5 μmor less.
 6. The medium of claim 1, wherein 0.9≦M/U≦1.2.
 7. The medium ofclaim 6, wherein the magnetic layer has a thickness of 0.5 μm or less.8. The medium of claim 1, wherein the amount of liquid lubricantactually present within the lubricant-containing coating layer is in therange from 50% to 80% of U.
 9. The medium of claim 8, wherein themagnetic layer has a thickness of 0.5 μm or less.
 10. The medium ofclaim 1, wherein the magnetic layer has a thickness of 0.5 μm or less.11. A magnetic recording medium comprising a nonmagnetic substrate, aliquid lubricant-containing coating layer formed on the substrate, and amagnetic layer formed directly on the coating layer, wherein the maximumamount of liquid lubricant which can be absorbed by a layer per unitvolume of the layer is defined by U for the lubricant-containing coatinglayer and M for the magnetic layer, wherein 0.75≦M/U≦1.25, and theamount of liquid lubricant actually Present within thelubricant-containing coating layer is in the range from 35% to 85% of U.